Because money is a big issue.

There are plenty of real studies, not just this one, that people's happiness dramatically increases with money, up to a plateau, past that plateau happiness doesn't increase.

Last I checked, I think it was 70-80K Salary was a baseline. Below that, yes, happiness was really impacted by being without money.

And since this one was 'generic', across the population, and there are a lot of people <80K salaries, then yes, it is a big variable.